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MEAs for PEM fuel cells are typically made by the pressing of the membrane between the electro- des layers (catalyst and gas diffusion layers) and then by the sealing of the MEA between the bipo- lar plates that supply the gases to the fuel cell. All this is done while the membrane is dry. In the fuel cell, water is absorbed into the membrane from the feed streams. Membrane hydration is accom- panied by membrane swelling. The membrane swelling creates a pressure against the electrode layers and bipolar plates. The swelling pressure of the membrane is the result of the increase in energy by water sorption into the membrane. A simple energy balance shows that the swelling pressure (Pswelling) is the energy of water sorption, DHabsorption, divided by the change in volume asso- ciated with the water sorption VH2O. water absorption is the solvation energy. By con- straining the swelling, we are looking at the water absorption energy for the initially sorbed water, which should correspond to ionization of the acid moieties. The enthalpy of ionization is not very sensitive to temperature. The swelling pressure was used to determine an enthalpy of water sorp- tion of 20 6 3 kJ/mol. To obtain this value, it was assumed that the partial molar volume of water in Nafion was equal to the molar volume of water. This enthalpy of water absorption is comparable to that obtained by Morris and Sun12 and by Escoubes and Pineri.52 CONCLUSIONS The mechanical and electrical properties of Nafion and Nafion/TiO2 composite membranes were examined; we focused on the properties of these materials in constrained environments. The key results from these studies are as follows: 1. Composite membranes have a lower resistiv- ity than Nafion under fully humidified condi- tions and give better iv performance in fuel cells. 2. Water sorption into Nafion-based mem- branes increases with increased tempera- ture. Recast membranes sorbed water faster than extruded Nafion. 3. The elastic modulus of dry Nafion and Nafion/TiO2 composite membranes decreases from300MPaat258Cto80MPaat908C and then decreases to 2–3 MPa at 110 8C. 4. Water sorption is a balance between the energy of solvation of the sulfonic acid groups and the energy to swell the membrane. The reduction of the elastic modulus with in- creased temperature reduces the energy re- quired to swell the membrane and permits greater water sorption. 5. Water plasticizes Nafion and Nafion/TiO2 membranes. At room temperature, the elas- tic modulus decreased from 300 MPa for dry membranes to 50 MPa for fully humidified membranes. The elastic modulus of compos- ite membranes did not decrease as much with the water content as Nafion membranes did. 6. Water reduced the plastic modulus of Nafion/ TiO2 membranes more than that of Nafion. 7. Nafion/TiO2 membranes crept 40% less than Nafion membranes did at 25 8C and 100% RH after 3 h. ðEnergy of water absorptionÞ Pswelling 1⁄4 ðVolume change on absorptionÞ 1⁄2SO#H O=SO1⁄2H 3 2 3 absorption 1⁄4 1⁄2SO#H2O=SO1⁄2V"H O 332 ð10Þ TITANIA/NAFION COMPOSITE MEMBRANES 2343 Equation 10 is a simple statement that the swelling pressure is the ratio of the enthalpy of water sorp- tion divided by the partial molar volume of water in the membrane. The swelling pressure decreases as the water content increases because the heat of sorption of water decreases with the water content. Figure 14 shows the dynamic measurement of the swelling pressure of Nafion 115. Just after the water injection, the force increased, went through a maximum 2000 s after water injection, and then relaxed over a time of 50,000 s. The rise in the force after water injection corresponds to the water sorp- tion into the membrane. The dynamic water uptake measurements (Figs. 10 and 11) show that it took 1000–4000 s for water sorption to be equilibrated, which is the same time observed for membrane swelling. The long time relaxation out to 50,000 s is the result of polymer creep from the applied stress of the swelling pressure. The dynamics of the swel- ling pressure measurement should be representa- tive of what happens in a fuel cell when the mem- brane is hydrated: it swells, creating a stress, and subsequently creeps because of the applied stress. The surprising result from the swelling pres- sure measurements was that the swelling pressure showed little or no dependence on temperature, whereas all the other mechanical properties and water uptake varied substantially with tempera- ture. According to eq 10, the swelling pressure depends on the enthalpy of water absorption and the partial molar volume of water. The enthalpy of Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polbPDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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